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Lundy, Isle of Avalon

Sea -Levels 

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Sea -Levels

Global sea levels rose by an average 2mm per year in the 20th century with some predictions for the 21st century showing a rate double that. The rise in sea level has two main effects; the most obvious of which is flooding of lowlands, but equally important is the erosion of coastal beaches and bluffs. A sea level rise of 2mm per year will cause a 2 metre horizontal beach loss over a decade.

'For the British Isles mean sea level rose by about 1.5mm per year in the 20th century but is now rising more slowly than it did over a base period of 1921 to1990. Sea levels are predicted to rise by between 2 and 9mm per year (a rise in sea level of between 12cm and 67cm by the 2050s) due to climate change. This represents a tripling in the rate of rise experienced in the 20th century'. - Source Environment Agency

'According to satellite observations the world's sea level has been rising at the rate of 3.4mm per year for the last fifteen years and that the rate of rise is increasing........ the global  sea level is estimated to keep on rising and by 2100 it will have risen by more than one metre, perhaps by as much as two. ....  Atlantic - The Biography of an Ocean - Simon Winchester - 2010

 

 

Glaciation

 There have been several occasions throughout history when sea-levels have risen, and fallen, by significantly greater amounts in short periods of time. A good example of this was during the post-glaciation phase after the last Ice-age.

Researchers have used various methods to calculate the glacio-eustatic fluctuations in the sea level during the last glaciation, but there is no standardisation in the magnitude and the dynamics of their calculations. However according to the estimates of most researchers the sea levels during the last glaciation (18-16 thousand years ago) was in the region of 100-170 metres lower than at present:

 

Glacio-eustatic fluctuations of the mean sea level

During the last Ice-age much of Northern Europe was covered by glaciers, the so-called Scandinavian ice sheet. In addition to absorbing vast amounts of water, the great weight of this mass of ice isostatically depressed the land underneath. For example, in the British Isles the consequence of this was that the North of Britain was depressed/sank under the weight of the ice while as a result of isostastic balancing, the south of Britain was elevated.

This effect reversed itself ( Post Glacial Rebound ) when the ice melted during the post-glaciation period. Without the weight of the ice the land rose in Scotland and conversely the south-west of Britain sank at the same time as the sea level rose. According to some estimates the magnitude of these changes could have been in the region of 100m.

Millennia after the end of the Ice age the effects of PGR continue to be felt across the entire planet. Despite a global mean rise in sea levels of 2mm/year, a rise of 3.5mm has been recorded at Baltimore, Maryland and a fall in sea level of 4mm/year has been recorded at Stockholm, both reflecting PGR. PGR means that sea level change is not uniform as some parts of the land mass rise and others fall.

 

Sea Levels in the South-West of Britain

 

'The coastline of Britain is fairly young in geological terms; as recently as 20,000 years ago the sea was thought to be about 100 to 150m below its present level. It was only after the most recent ice age, about 8,000 to 15,000 years ago, that sea levels rose to inundate low-lying land, forming the North Sea and English Channel. After this inundation, sea level rise slowed down and has remained relatively constant for the last 6,000 years.'  - Environment Agency

The effect of the flooding caused by the melting ice was exacerbated by the isotatic changes caused by post glacial rebound (PGR) as the weight of the ice sheet melted away. Nearly 10,000 years later this effect is still felt. Currently Scotland is rising at a rate of 1mm/year relative to the south of England.

The flooding from the melting ice must have happened relatively rapidly, while the PGR would probably have happened strongly at first and then more slowly, quite possibly with some violent seismological effects.

'Large, unpredictable fluctuations in sea level change that persist over decades are commonplace' [Douglas, 1992].

''If we turn to the Bristol Channel,' asserted the eminent geologist Sir Charles Lyell,' we find that both on the north and the south sides of it there are numerous remains of submerged forests; .. one of those at Porlock Bay ... extends far from the land. There is good reason to believe that there was once a woodland tract uniting Somersetshire and Wales, through the middle of which the ancient Severn flowed. This estuary of the Severn is in reality the largest 'drowned valley' in Britain. The outline of it has been somewhat modified by tidal action, and erosion continues at several sites.

'..geologists have found beaches which were raised dozens of feet above high water mark and some of these have later been returned to sea level. There a re also submerged forests to be found off  the coast of Cornwall.' --Mysterious Britain.

'According to data in 'The Subsidence of London' published in 1932, borings taken in the lower part of the Thames valley show that the land surfaces in late Neolithic times, about 2000 BC [the era of Stonehenge ], now lie between 60 and 70 feet below mean sea level. Even Roman remains under London lie about six feet below the Trinity high water mark. These discoveries mean that the whole geography of Britain was considerably different in ancient times when the great Megalithic monuments like Avebury, Stonehenge etc. were constructed.

' The Roman historian, Marcelinus, who lived in the fourth century of the Christian Era, described the swallowing up of 'a large island' in 'the Atlantic Sea'. This has always been identified as Atlantis, though it may equally be a record of the disintegration of islands on the coast of Britain, including parts of the Scillies, which probably went down during his lifetime.'  From Lost Lands and Sunken Cities

'The drowning of Mounts Bay is just one instance of a phenomenon visible all round the south - western promontory of Britain, once known as Dumnonia. In his 'Report on Cornwall', Sir Henry de la Beche remarked that 'submarine forests are so common that it is difficult not to find traces of them in the district at the mouths of all the numerous valleys which open upon the sea and are in any manner silted up'.

'All the evidence of geologists, antiquaries and archaeologists points to a permanent incursion of the sea having taken place at some time after the Roman period but before the end of the Middle Ages. The submerged coastline of the former Dumnonia, complete with its drowned valleys and forests, indicates some kind of major subsidence, perhaps as the result of an earthquake, must have occurred.'   From Lost Lands and Sunken Cities

Large tracts of coastal land have vanished from mainland Britain over the last two thousand years.

 

'Ptolemy's map of the British Isles, for instance, whilst considerably inaccurate for the coast of Scotland, tallies reasonably well with the rest of the country, and has the added bonus of possessing a record of longitudes and latitudes for important promontories. Anglesey, which in Roman times was separated from the mainland only by a fordable creek. Cornwall, too, is shown considerably larger than at present

It is recorded that as late as the fourth century, the Scillys, now an archipelago of many islets, was but a single large island. The Scillys.. "were once connected by the now submerged sea-bed to Cornwall." - Snyder - Sub-Roman Britain.  In the year 387 Emperor Maximus exiled two Priscillianist heretics to Scilly.

"Celestial phenomena appear the same pretty much the world over, with a few exceptions. But, if a man in fifth century China saw a comet, chances are good people in fifth century Britain saw it, too. These astronomical records include, around the fifth century, mention of "dragons" coming out of the heavens and smashing into the earth. At about the same time, the Anglo-Saxon Chronicle records how the sun was "dimmed," or how the "stars showed fully nearly half an hour past nine in the morning." At about the same time, tree ring samples from Ireland show that their growth was stunted about the fifth century AD, possibly from lack of sunlight. Saint Patrick, who lived at about the same time, spoke of apocalyptic weather patterns "  - 

In "The Age of Arthur," John Morris notes, "There is ample evidence for a drastic change in sea level of Europe and the Mediterranean towards the end of the Roman Empire; its severity affected the coast and rivers of southern and eastern Britain."

'Many records are also preserved in a curious compilation, published in 1749 by Dr. Thomas Short, F.R.S. Titled 'A General Chronological History of The Air, Meteors, Weather, Seasons, etc., the treatise compiled during sixteen years of research, lists many strange and extreme events. Among them are the great earthquakes of Wales in AD 394, which 'made sad havoc', and the Cornish earthquake of AD 424, where there were 'great losses, many killed'.

 

Rhys in Celtic Folklore discusses the many Welsh legends concerning inundations [he mentions a tidal wave in the early part of 1607].[p.426]

'It is interesting to note the fact that Celtic folklore connects the underground divinities intimately with water.

 

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